Chapter 2 – PPP CCNA ppsx

Introducing Serial Communications• Computers make use of relatively short parallel connections between interior components, but use a serial bus to convert signals for most external comm

Chapter 2 – PPP

CCNA Exploration 4.0

Introduction

Serial Point-to-Point Links

Introducing Serial Communications

• Computers make use of relatively short parallel connections between interior components, but use a serial bus to convert signals for most external communications.

Introducing Serial Communications

• In a parallel connection, it is wrong to assume that the 8 bits leaving the sender

at the same time arrive at the receiver at the same time Rather, some of the bits get there later than others This is known as clock skew

Introducing Serial Communications

• Parallel wires are physically bundled in a parallel cable, and signals can imprint themselves on each other The possibility of crosstalk across the wires

requires more processing, especially at higher frequencies

Introducing Serial Communications

• Serial Communication Standards

– All long-haul communications and most computer networks use serial connections, because the cost of cable and synchronization difficulties make parallel connections impractical

– The most significant advantage is simpler wiring Also, serial cables can be longer than parallel cables, because there is much less interaction (crosstalk) among the

Introducing Serial Communications

• Time Division Multiplexing

– Bell Laboratories invented time-division multiplexing (TDM) to maximize the amount of voice traffic carried over a medium.

– TDM divides the bandwidth of a single link into separate channels or time slots

• Statistical Time Division Multiplexing (STDM)

– STDM uses a variable time slot length allowing channels to compete for any free slot space It employs a buffer memory that temporarily stores the data during periods of peak traffic

– STDM does not waste high-speed line time with inactive channels using this scheme

TDM Examples

TDM Examples

Demarcation Point

• The CPE, which is generally a router, is the DTE The DTE could also

be a terminal, computer, printer, or fax machine if they connect directly

to the service provider network

• The DCE, commonly a modem or CSU/DSU, is the device used to

convert the user data from the DTE into a form acceptable to the WAN service provider transmission link This signal is received at the remote DCE, which decodes the signal back into a sequence of bits The

remote DCE then signals this sequence to the remote DTE.

Cable Standards

• The DTE/DCE interface for a particular standard defines the following

specifications:

– Mechanical/physical - Number of pins and connector type

– Electrical - Defines voltage levels for 0 and 1

– Functional - Specifies the functions that are performed by assigning

meanings to each of the signaling lines in the interface

Cable Standards

Cable Standards

Parallel to Serial Conversion

• Your PC also has a Universal Asynchronous Receiver/Transmitter (UART) chip

on the motherboard Since the data in your PC flows along parallel circuits, the UART chip converts the groups of bits in parallel to a serial stream of bits

• To work faster, a UART chip has buffers so it can cache data coming from the system bus while it processes data going out the serial port

• The UART is the DTE agent of your PC and communicates with the modem or other serial device, which, in accordance with the RS-232C standard, has a complementary interface called the DCE interface.

HDLC Encapsulation

• HDLC - The default encapsulation type on point-to-point connections,

dedicated links, and circuit-switched connections when the link uses two Cisco devices HDLC is now the basis for synchronous PPP used

by many servers to connect to a WAN, most commonly the Internet.

HDLC Encapsulation

Configuring HDLC Encapsulation

• Cisco HDLC is the default encapsulation method used by Cisco

devices on synchronous serial lines

• You use Cisco HDLC as a point-to-point protocol on leased lines

between two Cisco devices If you are connecting to a non-Cisco

device, use synchronous PPP.

Troubleshooting a Serial Interface

• You can identify any of the following five possible problem states in the interface status line:

1 Serial x is down, line protocol is down

2 Serial x is up, line protocol is down

3 Serial x is up, line protocol is up (looped)

4 Serial x is up, line protocol is down (disabled)

5 Serial x is administratively down, line protocol is down

Troubleshooting a Serial Interface

Troubleshooting a Serial Interface

• The command syntax varies, depending on the platform Cisco 7000 series routers use a cBus controller card for connecting serial links

With these routers, use the show controllers cbus command

Troubleshooting a Serial Interface

Troubleshooting a Serial Interface

Troubleshooting a Serial Interface

Troubleshooting a Serial Interface

Troubleshooting a Serial Interface

• Activity 2.1.7.3

Troubleshooting a Serial Interface

• Activity 2.1.7.3

Troubleshooting a Serial Interface

• Activity 2.1.7.3

Troubleshooting a Serial Interface

• Activity 2.1.7.3

PPP Concepts

Introducing PPP

• What is PPP?

– Cisco HDLC is the default serial

encapsulation method when you

connect two Cisco routers

– When you need to connect to a

non-Cisco router, you should use

PPP encapsulation.

– PPP establishes a direct

connection using serial cables,

phone lines, trunk lines, cellular

telephones, specialized radio links,

• PPP supports PAP and CHAP

authentication This feature is explained and practiced in a later section.

PPP contains 3 main components:

1 HDLC protocol for encapsulating datagrams over point-to-point links

2 Extensible Link Control Protocol (LCP) to establish, configure, and test the data link connection

3 Family of Network Control Protocols (NCPs) for establishing and

configuring different network layer protocols

PPP Architecture - Link Control Protocol Layer

• The LCP provides automatic configuration of the interfaces at each end, including:

– Handling varying limits on packet size

– Detecting common misconfiguration errors

– Terminating the link

– Determining when a link is functioning properly or when it is failing

• PPP also uses the LCP to agree automatically on encapsulation formats (authentication, compression, error detection) as soon as the link is established

PPP Architecture - Network Control Protocol Layer

PPP Frame Structure

Establishing a PPP Session

• Phase 1: Link establishment and configuration negotiation

• Phase 2: Link quality determination (optional)

Extra: PPP Session Establishment (Detail)

1 Link establishment - (LCPs)

2 Authentication - Optional (LCPs)

3 Link quality determination - Optional (LCPs)

4 Network layer protocol configuration (NCPs)

5 Link termination (LCPs)

Establishing a Link with LCP

• LCP operation uses three classes of LCP frames to accomplish the work of each of the LCP phases:

– Link-establishment frames establish and configure a link (Configure-Request,

Configure-Ack, Configure-Nak, and Configure-Reject)

– Link-maintenance frames manage and debug a link (Code-Reject, Protocol-Reject,

Establishing a Link with LCP

LCP Packet

LCP Packet

PPP Configuration Options

• PPP can be configured to support various functions including:

1 Authentication using either PAP or CHAP

2 Compression using either Stacker or Predictor

PPP Configuration Options

NCP Explained

NCP Explained

• Activity 2.2.6.2

NCP Explained

• Activity 2.2.6.2

NCP Explained

• Activity 2.2.6.2

Configuring PPP

PPP Configuration Commands

– R3(config-if)#compress [predictor | stac]

• Example 3: Link Quality Monitoring

Extra: More Information on Compression

Cisco supports these types of compression:

 Predictor-Determines whether the data is already compressed If

so, the data is just sent-no time is wasted trying to compress

already compressed data

 Stacker-A Lempel-Ziv (LZ)-based compression algorithm looks at

the data, and sends each data type only once with information

about where the type occurs within the data stream The receiving side uses this information to reassemble the data stream

 MPPC-This protocol (RFC 2118) allows Cisco routers to exchange

compressed data with Microsoft clients MPPC uses an LZ-based compression algorithm

 TCP header compression-This type of compression is used to

compress the TCP headers

Extra: More Information on Compression

Important notes on compression:

– The highest compression ratio is usually reached with highly compressible text files

– Already compressed files such as JPEG graphics or MPEG files, or files that were compressed with software such as PKZIP or StuffIt, are only

compressed 1:1, or even less.

– Trying to compress already compressed data can take longer than

transferring the data without compression

– Compressing data can cause performance degradation because it is

software, not hardware compression

– Compression can be CPU or memory intensive.

– Predictor is more memory intensive and less CPU intensive, whereas

Stacker and MPPC are more CPU intensive and less memory intensive

Memory intensive means that an extra memory allowance is required.

Extra: Enable Link Quality Monitoring (LQM)

• Link Quality Monitoring (LQM) is available on all serial interfaces running PPP LQM will monitor the link quality, and if the quality drops below a configured percentage, the router shuts down the link

• The percentages are calculated for both the incoming and outgoing

directions

– The outgoing quality is calculated by comparing the total number of packets and bytes sent with the total number of packets and bytes received by the destination node

– The incoming quality is calculated by comparing the total number of packets and bytes received with the total number of packets and

bytes sent by the destination peer.

• When LQM is enabled, Link Quality Reports (LQRs) are sent, in place of keepalives, every keepalive period All incoming keepalives are

responded to properly If LQM is not configured, keepalives are sent

every keepalive period and all incoming LQRs are responded to with an LQR.

• LQR is specified in RFC 1989, PPP Link Quality Monitoring, by William

A Simpson of Computer Systems Consulting Services.

Verifying a Serial PPP Encapsulation Configuration

Troubleshooting PPP Encapsulation

Troubleshooting PPP Encapsulation

Troubleshooting PPP Encapsulation

Troubleshooting PPP Encapsulation

Configuring PPP

with Authentication

PPP Authentication Protocols

• The authentication phase of a PPP session is optional If used, you can authenticate the peer after the LCP establishes the link and choose the authentication protocol If it is used, authentication takes place before the network layer protocol configuration phase begins

Password Authentication Protocol (PAP)

Challenge Handshake Authentication Protocol (CHAP)

•

PPP Encapsulation and Authentication Process

Configuring PPP with Authentication

Notes:

- sent-username and password must match remote username and password

Example: Configuring PAP

172.25.3.0/24

.2/S0

DCE DTE

interface Serial0

ip address 172.25.3.1 255.255.255.0 encapsulation ppp

ppp authentication pap ppp pap sent-username HQ

password HQpass

Extra: One-way PAP authentication

(config)# hostname RA

(config)# interface serial 0

encapsulation ppp ppp pap sent-username RA-X password CISCO-X

(config)# hostname RB (config)# username RA-X password CISCO-X (config)# interface serial 0

encapsulation ppp ppp authentication pap

Extra: Two-way PAP authentication

(config)# hostname RA

(config)# username RB-X password CISCO-B

(config)# interface serial 0

encapsulation ppp ppp authentication pap ppp pap sent-username RA-X password CISCO-A

(config)# hostname RB (config)# username RA-X password CISCO-A (config)# interface serial 0

encapsulation ppp ppp authentication pap

Notes:

- Hostnames are involved unless the ppp chap hostname command is used, and

must match remote router’s username command (not case-sensitive)

Example: Configuring CHAP

172.25.3.0/24

.2/S0

DCE DTE

hostname SantaCruz

username HQ password boardwalk

ppp chap hostname SantaCruz (optional)

username SantaCruz password boardwalk

ppp chap hostname HQ (optional)

interface Serial0

ip address 172.25.3.1 255.255.255.0 encapsulation ppp

ppp authentication chap

Extra: One-way CHAP authentication

(config)# hostname RA-X

(config)# username RB password CISCO

(config)# interface serial 0

encapsulation ppp OR

(config)# hostname RA

(config)# interface serial 0

encapsulation ppp ppp chap hostname RA-X ppp chap password CISCO

(config)# hostname RB (config)# username RA-X password CISCO (config)# interface serial 0

encapsulation ppp

Troubleshooting a PPP Configuration with Authentication

• In the last line, the code = 4 means a failure has occurred Other code values are as

Extra: Configuring PPP Multilink (MLP)

Router(config)#interface serial 0/0

Router(config-if)#encapsulation ppp

Router(config-if)#ppp multilink

• In some environments, it may be necessary to bundle multiple serial

links to act as single link with aggregated bandwidth

• Multilink PPP provides load balancing over the router interfaces that

PPP uses.

Extra: Configuring PPP Multilink

interface loopback 0

ip address 192.168.1.2 255.255.255.0

interface Virtual-Template1

ip unnumbered loopback0 ppp multilink

interface Serial0

no ip address encapsulation ppp ppp multilink interface Serial1

no ip address encapsulation ppp ppp multilink interface Serial2

no ip address encapsulation ppp

Extra: Configuring PPP Multilink

(config)# interface Multilink 1

ip address 1.2.3.4 255.255.255.0 ppp multilink

(config)# interface Serial 0/0

no ip address encapsulation ppp ppp multilink

ppp multilink group 1 (config)# interface Serial 0/1

no ip address encapsulation ppp ppp multilink

Labs

Summary